adum wrote:1. thanks. i have fixed this for the next version.2. i can see some advantages of selling sets by rank. however, that makes it more difficult for people who want a variety of problem difficulties. so there's no perfect solution. i picked this way.

Sure there is. You can compile different sets, sorted and by rank, and by category, and by author, for any customer. So what if these sets will overlap in some problems?

interesting idea. i'll think about that -- not sure if Apple would even allow it. let's say you first bought all the current sets. if you bought a set of say 4d problems, you'd be paying money for nothing.